1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus such as a copier, a printer, or a fax machine, and to an image forming apparatus including the fixing device.
2. Description of the Related Art
Recently, an image forming device, such as a copier, a printer, or a fax machine, forms images through an image forming process such as electrophotographic recording, electrostatic recording, and magnetic recording, and forms unfixed toner images by the image transfer method or the direct method on recording materials such as recording material sheets, printing paper, photosensitive paper, or electrostatic recording paper. A known example of a fixing device for fixing an unfixed toner image performs a fixing process by applying heat and pressure to the toner image formed on recording paper while placing the image at a nip part between an endless belt and a pressure roller.
In such a fixing device, heating the entirety of the belt has been made possible. Thus, the fixing device can have a reduced first print time from the heating standby time and can avoid shortage in the amount of heat at the high-speed rotation and attain a favorable fixing quality even when mounted on an image forming apparatus having high productivity.
A fixing device is built on the premise that various recording media pass through the device, and thus a recording medium the width of which is smaller than a heat generation width of a heater of the fixing member in the axial direction of the heater may pass therethrough, for example.
In the above-described fixing device, when such a recording medium passes, a region of the fixing member through which no sheet passes is not deprived of heat by the recording medium, thereby coming to have an excess amount of heat (a temperature increase at a no-sheet passing portion). This temperature increase accelerates deterioration of the fixing member, thereby making the life thereof shorter.
In response to this inconvenience, another fixing device has been developed to avoid a temperature increase at a no-sheet passing portion. This fixing device includes, as a nip-forming member, a heat-averaging member made of a material having high thermal conductivity and has the heat-averaging member in constant contact with the inner side of a fixing belt.
However, in the fixing device having the heat-averaging member in constant contact with the inner side of the fixing belt, the nip-forming member and the fixing belt make direct contact with and slide over each other. Therefore, the fixing device is disadvantageous in terms of durability and temperature stability of the fixing belt.
In order to overcome such disadvantages, another known fixing device includes a nip-forming member provided with a sliding sheet made of a low-friction material (refer to, for example, Japanese Patent No. 4818826). Specifically, a sliding sheet 601 is wound around a nip-forming member 600 as illustrated in FIGS. 24A and 24B. Thereafter, the sliding sheet 601 is fastened to the nip-forming member 600 by a screw 603 provided as a fastening member with a securing member 602 therebetween. Additionally, a plurality of projecting parts 600a are provided at the center in the width direction of the nip-forming member 600 and on one side thereof opposite to a nip-forming side thereof. The projecting parts 600a abut a reinforcing member (not illustrated) for supporting the nip-forming member 600. As illustrated in FIG. 25, the sliding sheet 601 overlaps with itself in a region thereof contacted by the securing member 602, and is fastened to the nip-forming member 600 by the fastening member 603. Furthermore, double-faced adhesive tape 604 is stuck on the nip-forming surface of the nip-forming member 600. This configuration makes it possible to have a fixing belt without disadvantages in term of durability and temperature stability.
In the fixing device disclosed in Japanese Patent No. 4818826, however, there has been a disadvantage associated with attachment of the sliding sheet to the nip-forming member, such that: fixation of the sliding sheet and arrangement of projecting parts are concentrated on one side surface of the nip-forming member opposite to the side thereof having the nip part; and it has therefore been difficult that, while the sliding sheet is fixed to the nip-forming member so as to appropriately make slidable contact with the fixing belt, the projecting parts are arranged so as to appropriately abut a reinforcing member. There has been another disadvantage such that, depending on how the sliding sheet is attached, the arrangement precision of the nip-forming member inside a unit may be reduced and the nip part is not appropriately formed. There has been still another disadvantage such that, since the sliding sheet is wound at least one turn around the nip-forming member in as direction of the short side thereof, the component cost for the sliding sheet is high and hinders cost reduction.
There is a need to provide a fixing device that, while having a simple structure, prevents temperature increase at a no-sheet passing portion and has a fixing belt the durability and temperature stability of which are ensured.